If you heat treat your draw bar, it will need to be tempered carefully or the thread will crumble, and if the tempered area stops abruptly you have a good chance of cracking. If you have to have other than mild steel, 4140 is what I would use, it can be heat treated, but in this case is ok as is, it takes a thread better than silver steel(thats what I'v found)
Ian S C